An electric linear motion actuator of this type includes a motion convertor mechanism which converts the rotary motion of the rotor shaft of an electric motor to a linear motion of an axially movable driven member.
Known motion convertor mechanisms used in this type of electric linear motion actuators include a ball-screw mechanism and a ball-ramp mechanism. These motion convertor mechanisms can increase power to some extent but cannot increase power to such an extent as required in an electric disk brake system.
Thus in an electric linear motion actuator using one of the above-mentioned motion convertor mechanisms, a speed reduction mechanism such as a planetary gear mechanism is additionally provided to increase driving force. Such a separate speed reduction mechanism adds to the complexity and the size of the electric linear motion actuator.
The applicant of the present invention has already proposed in JP 2009-197863A an electric linear motion actuator which is free of this problem, and which can sufficiently increase power without the need for a separate speed reduction mechanism and thus can be used in an electric disk brake system, of which the linear motion stroke is relatively small.
The electric linear motion actuator disclosed in JP 2009-197863A includes a rotary shaft rotated by an electric motor, an outer ring member provided around the rotary shaft, a plurality of planetary rollers mounted between the rotary shaft and the outer ring member, and a carrier rotatable about the rotary shaft and rotatably supporting the planetary rollers. When the rotary shaft rotates, the planetary rollers revolve around the rotary shaft while rotating about their respective axes due to frictional contact between the rotary shaft and the planetary rollers. A helical rib is formed on the radially inner surface of the outer ring member which engages in helical grooves or circumferential grooves formed in the radially outer surfaces of the respective planetary rollers such that the outer ring member and the carrier move in the axial direction relative to each other when the rotary shaft is rotated.
It is troublesome and thus costly to mount the planetary rollers between the radially outer surface of the rotary shaft and the radially inner surface of the outer ring member with an interference fit. Thus, in the electric linear motion actuator disclosed in JP 2009-197863A, the carrier supports roller shafts on which the respective planetary rollers are rotatably supported so as to be radially movable, and the roller shafts are all radially inwardly biased by elastic members, thereby elastically pressing the planetary rollers against the radially outer surface of the rotary shaft.
The elastic members used in the electric linear motion actuator disclosed in JP 2009-197863A are C-shaped rings each mounted around the roller shafts while kept in contact with the roller shafts to radially inwardly bias the roller shafts. The C-shaped rings are inexpensive but have the following problem to which no solution has yet been found.
Since the C-shaped rings each have circumferentially separate ends, when the roller shafts rotate even slightly about their respective axes due to rotation of the planetary rollers about their axes, the C-shaped rings move in the direction of the circle circumscribed about the roller shafts due to contact with the roller shafts, so that the C-shaped rings rotate in this direction until, eventually, one of the roller shafts radially aligns with and fits into the gap between the circumferentially separate ends of each C-shaped ring. This causes the C-shaped rings to radially shrink by their own elasticity. Once the C-shaped rings radially shrink, the C-shaped rings cannot press the planetary rollers against the radially outer surface of the rotary shaft any more, which in turn makes it impossible to transmit the rotation of the rotary shaft to the planetary rollers. In this state, the rotary shaft simply idles and the function of the electric linear motion actuator is lost.